You spent 10 years with a company, first of all, all you had was the experience in that company, you did not expose to the outside world

Not to sound rude but... what planet you on. I have plenty of experience with many different companies in the outside world. Just because I have worked at the same company for a long time does not mean I have tunnel vision and I have found on occasion I know more about how other companies work than their own employees do!

Second, over the 10 years in the company, what had this person done?

Oh! please, plenty and varied, a job and job title does not restrict your knowledge and experience only you do.

The same thing - backup, restore, install SQL Server, implement developement to production. I met a DBA working in a company for 10 years, he could not even write a decent DTS package and stored procedure.

That is a lack of knowledge and experience, nothing to do with loyalty.

Besides these days people changes job every 2 to 3 years is a norm. Actually those people I found had more business and technical experience

My experience is the opposite.

Especially if you are a contractor, you work in a company for six months and then another one for six months, it does not mean that person is not good.

Does not mean they are good either. I have met many, many contractors who knew less about databases than me and even when I knew nothing. And even now I know of contractors who have no breadth of knowledge due to specific restrictive contracts.

I am not saying that frequent movers are better or worse than long serving employees, only that you need to know the circumstances.